Ground proximity warning systems that have a terrain clearance mode which provides a pilot with a warning in the event that the aircraft should descend below a predetermined altitude above the ground are known. Examples of such systems are described in U.S. Pat. Nos. 3,946,358, 3,944,968 and 4,030,065. Such terrain clearance warning systems are generally not used alone, but rather are used in conjunction with other warning criteria to warn the pilot of other hazardous conditions, such as excessive closure rate with terrain, excessive sink rate, excessive descent after take off, excessive descent below the glide slope and descent below a preset minimum altitude to form a ground proximity warning system. These different warning criteria are often referred to as modes of operation. Not all of the modes of operation are generally enabled simultaneously, and mode switching is provided to switch to the appropriate mode or modes associated with a particular phase of flight.
A ground proximity warning system employing multiple warning modes is described in U.S. Pat. No. 3,946,358. The aforementioned patent discusses a negative climb after take-off mode, which is activated upon take off and generates a warning if the aircraft should descend excessively before reaching seven hundred feet of radio altitude. Also described is mode switching circuitry for switching between the negative climb after take-off mode and the terrain clearance modes. Other patents that disclose various warning modes and mode switching circuitry are U.S. Pat. Nos. 3,947,810; 4,319,218 and 4,433,323.
The above-described systems are designed for transport aircraft and utilize signals representative of landing gear and flap position to alter the warning criteria and effect mode switching. However, not all aircraft are operated as are transport aircraft, and consequently, flap and gear position may not be a reliable indicator of the phase of flight, particularly, in the case of military aircraft, such as, for example, the Lockheed S3-A. In addition, military aircraft have different flight and operational characteristics than transport aircraft, and additional sets of warning criteria must be provided to accommodate such aircraft and the ways in which they are operated.